In the fuel system of internal combustion engines, particularly diesel engines, one type of fuel system utilizes a fuel pump in which a valve connected to the throttle linkage determines the amount of fuel to be distributed by the pump to the fuel injectors of the engine. The throttle linkage is limited in its effect on the fuel valve by means of a governor which, through the action of centrifugally actuated flyweights or the like, limits the amount of fuel at higher speeds so that the engine will not exceed rated speed at rated power, nor will it exceed a high idle speed when the engine is unloaded. The throttle linkage is also limited in its effect on the fuel valve by an air/fuel ratio regulator which is connected to the engine air inlet manifold, and typically contains a bellows or diaphragm responsive to the pressure of air in the air inlet manifold so as to limit the amount of fuel applied to the engine to an amount which can be efficiently consumed with air supplied at the pressure in the air inlet manifold. Thus, as an engine starts to accelerate, the turbocharger has not built up full operating pressure in the air inlet manifold, so that the engine cannot efficiently utilize all of the fuel being called for by the throttle linkage; this lower pressure is sensed in the air/fuel regulator and limits the amount of fuel to be supplied to the engine. But as the engine builds up speed, the turbocharger compresses the inlet air to the desired operating pressure so that operation of the engine with the full fuel commanded for it will not be in an overfueled condition.
A typical air/fuel ratio regulator may have an extension of on the order of 50 mils between atmospheric pressure and maximum air intake manifold pressure; this will provide a commensurate control over a total throttle linkage travel of on the order of 300 mils. Typically, the air/fuel regulator will allow full fuel flow for air intake manifold pressures in excess of 18 psi, and provide a commensurate reduction therein for lower pressures.
One manner of testing the operation of the air/fuel regulator is to simply measure the extent of traverse thereof (in its effect on the throttle linkage) as a consequence of being connected to atmospheric pressure or to a known regulated pressure (such as shop air) which is in excess of its design (maximum-fuel) pressure (such as 18 psi). However, the measurement of such small distances (a portion of the 50 mils of its maximum travel) in other than a bench test environment is extremely impractical; and removal of the device for test is an undue burden and expense, and can result in creating other engine problems through mechanics errors and the like. To measure this travel electronically while the air/fuel ratio regulator is mounted on the engine would require the use of a precise linear motion transducer, such as the well known linear variable differential transformer (LVDT); however, such devices are not readily adapted for use in rugged environments and are otherwise unsuited to this purpose. And the cost and time, added to the test burden by the mounting of such a transducer, is to be avoided.